Elongated objects such as, for example, metallic wires, bars, rods or tubes can serve as starting materials for high quality end products and are frequently subject to extremely high quality requirements. Testing for material flaws, for example, for cracks near to the surface, cavities or other material inhomogeneities, forms an important part of the quality control of these products. In this context, as far as possible uninterrupted testing of the material surface with a high resolution is generally aimed at, which testing is to be carried out where possible at the manufacturing location in synchronism and at the speed of the manufacturing process. Nowadays such tests are often carried out using magnetic methods, in particular eddy current technology in a feed-through method. When testing using a feed-through method, an object which is to be tested (test specimen) is moved at a relatively high feed-through speed through a test section equipped with the corresponding sensor system, and the object is tested in the process.
One class of test apparatuses for the feed-through method has a test head which rotates about the object passing through and has test probes which are mounted thereon and which, given suitable matching of the rotational speed and the feed-through speed permit uninterrupted testing of the objects with a high spatial resolution. In another class of feed-through methods, feed-through coils, which surround the test specimen and through which the object to be tested is guided, are used.
During the non-destructive testing of materials using the eddy current method, an exciter coil is used to generate an electrical alternating current (eddy current) with a suitable direction, magnitude and frequency in the material to be tested, and the irregularities which are produced in the eddy current are detected and evaluated using sensors, for example, a coil arrangement. In eddy current testing, use is made of the effect according to which most impurities or defects in an electrically conductive material have a different electrical conductivity and/or different permeability from that of the test material itself. The measurement signal to be evaluated is determined in particular from the conductivity and permeability of the material of the test specimen and from the distance between the eddy current sensor and the surface of the material, wherein the absolute strength of the fault signal and also the ratio between the useful signal and interference signals (signal-to-noise ratio) decrease as the distance of the sensor from the surface of the material increases.
To compensate for the strong influence of the distance between the sensor and the surface of the material on the measurement signal, various systems for distance compensation have been proposed for test apparatus with rotating heads (such as, for example, DE 40 03 330 A1). The influence of the distance of the test coils, arranged at a distance from the item to be tested, corresponds to the so-called “fill factor” of the feed-through coil in the methods which use feed-through coils. What is referred to as the fill factor is generally understood to be the ratio of the cross section of the test material to the effective coil cross section. It is generally observed that the fault signal amplitude decreases the smaller the fill factor. For this reason, attempts are made to fill the cross section of the feed-through coil as far as possible by the test specimen, with the result that the inner regions of the feed-through coils extend relatively close to the surface to be tested. However, particularly in tests with a relatively high fill factor of the feed-through coil, relatively high interference levels which adversely affect the measuring accuracy are frequently observed. It has been observed that this effect can occur particularly in relatively thin test materials such as, for example, wires or thin bars.
It could therefore be helpful to provide a test method for testing elongated objects using feed-through coils, which test method permits testing with a high flaw signal level and at the same time a low interference level, in particular even in relatively thin test specimens such as wires or thin bars. It could also be helpful to provide a test apparatus which is suitable for carrying out the method.